Technical Field of the Invention
The present invention is directed to controlling the penetration of instruments such as drills and needles being inserted into either hard or soft material. Specifically, the invention is directed to penetrating a material layer of unknown thickness without plunging into the adjacent material layer. In medical applications, the invention allows for drilling through bone without plunging into the adjacent soft tissue or for inserting a needle through one tissue layer (e.g. skin) without plunging into the adjacent softer tissue or body cavity.
Description of the Prior Art
In general, there are three forms of controllable drills found in the prior art: mechanical-control drills, electrical-control drills and ultrasonic drills. In medicine, cranial drills are used to drill through patients' skull to give direct access to the brain. Current cranial drills can be further divided into three categories: Mechanical-control drills (Drills without electronic control circuits; can be with or without auto-stop mechanism); Electrical-control drills (Drills that are automatically stopped with the use of sensors, presumably with control feedback); and Ultrasonic drills (Drills that vibrate a stationary or rotational cutting tool at a high frequency in the axial direction to chip away at hard surfaces, but have no effect on soft materials).
Mechanical Drills
Currently, physicians are using a variety of cranial drills in surgery. The drill can be manual or powered, with or without automatic stop. An experienced physician can, in general, safely use a drill without auto-stop mechanism, so normal drills are still popular in hospitals (although even the most experienced clinicians could make mistakes). Nevertheless, drills with safety mechanisms are necessary for areas without trained neurosurgeons to reduce the possibility of mistakes during operations. U.S. Pat. No. 2,842,131 appears to be the oldest patent about the automatic stop cranial drill, followed by U.S. Pat. Nos. 4,456,010, 4,699,550, 4,803,982, 4,362,161, 4,600,006, and D596,743. Besides these, there are also other ways to make a drill safe: U.S. Pat. No. 5,382,250 uses an external stop to prevent drills from penetrating too far;
U.S. Pat. No. 2,842,131, entitled “Automatic Drill” describes a system of two concentric drill bores that have a clutch allowing them to spin in unison or not. The inside bore rotates and moves axially whereas the outer bore does not. This allows the inside bore to be compressed, engaging the rotational drive train, and the outer bore spins to create a shelf in the bone. When the inner bore breaks through the surface of the skull the clutch mechanism is released and both bores stop spinning. At this point, the outer bore rests on the shelf and the inner bore cannot proceed any farther. Products embodying this design are available from Acra-Cut. This product is used widely in operating room procedures where clinicians need access to the inside of the skull, but there are several limitations:
The size of the drill bits available by Acra-Cut are extremely limited. The outer bore cuts away extra bone unnecessarily. Further, the Acra-Cut tool must be powered by a pneumatic drill, which connects to an air hose and air compressor. Furthermore, if drilling with the Acra-Cut tool stops before penetration through the bone, then the rotation cannot recommence and thus a surgeon has to find an alternative means to finish penetrating the skull. These items are not easily made portable in emergency situations outside of the controlled operation room.
Additional patents using the same or similar concept of concentrically rotating drill bores that create a shelf for safety and contain a clutch mechanism include U.S. Pat. Nos. 4,362,161, 4,600,006, 4,803,982, 4,884,571, 5,135,532.
U.S. Pat. No. 5,382,250, entitled Cranial Drill Stop, describes a hard stop that is set to the correct depth based on the thickness of the skull at the point of drilling. The interlocking spacers will make sure that only the proper length of drill bit is exposed. This could be a very effective safety method if the exact skull thickness is known at a certain anatomical location a priori. However skull thickness has been shown to vary widely across individual skulls and across race and gender. Thus if the stop is not adjusted correctly, it is not effective and potentially dangerous.
U.S. Pat. No. 6,716,215, entitled Cranial drill with sterile barrier, describes a sterile barrier system that can protect an MRI-compatible drill from coming in contact with patient fluids. The drill described in the patent is gas powered, much like the Acra-Cut, so that it cannot be used in emergency settings.
Currently there are no small caliber drills which have an automatic stop after penetrating bone. The only drill that stops automatically is the Acra-Cut, which cannot be used for small hole penetration. It is designed to create larger holes in the skull and there are many important procedures for which large holes are not necessary.